1. Field of the Invention
The present invention relates to a motor for driving magnetic disks such as a spindle motor used in the hard disk drive device of the computer.
2. Description of the Related Art
Recently, the field of the hard disk drive device has been making steady progress in increasing capacity thereof. In order to optimize such a progress in increasing capacity, there is a growing need for higher rotational speed for the motor used in the hard disk drive device. As a bearing for such a motor, a ball bearing has been generally used so far. However, in order to optimize the need for higher rotational speed, application of fluid dynamic bearings has been introduced.
As an example of the motor used in the hard disk drive device and comprising a fluid dynamic bearing, there is shown in FIG. 20 a spindle motor for driving magnetic disks. The spindle motor 1 for driving magnetic disks (hereinafter, referred to as a spindle motor) is provided with a magnet 5 on the rotor 4 so as to face toward the stator 3 provided on the flange 2.
The flange 2 generally comprises a flange body 6 for holding the stator 3, and a sleeve 7 to be press-fitted into the hole (sleeve fitting hole 6a) formed on the flange body 6.
The sleeve 7 generally comprises a cylindrical sleeve body 9 and a disk-shaped counter plate 11.
The sleeve body 9 comprises a hole (no reference numeral is assigned) extending from one side (the upper side in FIG. 20) to the other side (the lower side in FIG. 20) for inserting a shaft 12 therein, and the hole is constructed of a hole formed on one side (hereinafter, referred to as a sleeve hole) 7a and an annular stepped portion 8 formed concentrically and in communication with the sleeve hole 7a via a step.
As shown in FIG. 21 and FIG. 22, the annular stepped portion 8 comprises an annular hole 8a having a larger inner diameter in comparison with the sleeve hole 7a and formed in communication with the sleeve hole 7a via a step (hereinafter, referred to as a medium diameter annular hole), and an annular hole having a larger inner diameter in comparison with the medium diameter annular hole 8a and formed in communication with the medium diameter annular hole 8a via a step (hereinafter, referred to as large diameter annular hole). The large diameter annular hole 8b opens at one end (the lower side in FIG. 21) of the sleeve body 9. The counter plate 11 is disposed at the large diameter annular hole 8b, and the counter plate 11 and the sleeve body 9 are hermetically connected by welding or the like.
The shaft 12 comprises a shaft body 12a, and an annular body 10 fitted on one end (the lower portion in FIG. 20) of the shaft body 12a The annular body 10 of the shaft 12 is disposed in the medium diameter annular hole 8a and the shaft body 12a of the shaft 12 is inserted into the sleeve hole 7a. 
As described above, the annular body 10 of the shaft 12 is disposed in the medium annular hole 8a and the shaft body 12a of the shaft 12 is inserted into the sleeve hole 7a, and the sleeve 7 constitutes a fluid dynamic bearing 13 with the shaft 12. Though oil 14 is generally used as a fluid for the fluid dynamic bearing 13, it may be constructed to use gas such as air.
In other words, a plurality of rows of grooves 15 are formed on the inner wall (sleeve hole 7a) of the sleeve body 9, and a plurality of rows of grooves (not shown) are formed on the end portion of the annular body 10 that touches the stepped wall surface of the medium annular hole 8a of the sleeve body 9 and the portion of the upper surface of the counter plate 11 that touches the annular body 10. Oil 14 is filled and reserved in the gap between the sleeve 7 including the grooves 15 and the shaft 12, and in the grooves that are not shown in the figure. The inner peripheral surface of the annular body 10 is formed with a fluid circulating groove 10a so as to facilitate circulation of the fluid. The annular body 10 slightly projects toward the counter plate 11 with respect to the shaft 12, so as to facilitate inflow and outflow of fluid from and to the fluid circulating groove 10a. 
The annular body 10 of the shaft 12 is disposed at the medium diameter annular hole 8a, that is, between the wall surface of the medium diameter annular hole 8a that faces in the axial direction (the upper side in FIG. 20) and the counter plate 11, so that the axial movement (vertical movement in FIG. 20) of the shaft 12 is controlled via the annular body 10.
The dynamic pressure generated by the pumping action in association with rotation of the shaft 12 forces a fluid layer to be formed between the sleeve 7 and the shaft 12, and the shaft 12 that touched the counter plate 11 as shown in FIG. 21 during the rest time rises from the counter plate 11 as shown in FIG. 22, so that the shaft 12 can rotate with respect to the sleeve 7 via the fluid layer. The fluid dynamic bearing 13 forms a fluid layer by the dynamic pressure and forms a gap between the shaft 12 and the counter plate 11 to support a thrust load of the shaft 12 as described above [in other words, the counter plate 11 supports a thrust load applied downwardly of the shaft 12 (in the direction of the arrow D in FIG. 20), and the ceiling wall of the medium diameter annular hole portion 8a supports a thrust load applied upwardly of the shaft 12 (annular body 10)(in the direction of the arrow U in FIG. 20)], and a radial load of the shaft 12 is supported by the portion of the sleeve 7 where the sleeve hole 7a is formed.
Referring now to FIG. 21 and FIG. 22, the operation of the fluid dynamic bearing of the related art will be described.
FIG. 22 shows a state in which the shaft 12 is rotated and the dynamic pressure of a fluid is generated.
In FIG. 22, when the spindle motor 1 is actuated and the shaft 12 starts rotating, the dynamic pressure is generated and thus a fluid layer is formed in the gap formed between the inner diameter surface of the sleeve 7 that is a fixed body and the outer peripheral surface of the shaft 12 that is a rotating body, between the stepped end surface (annular stepped portion 8) of the sleeve 7 and the opposing end surface of the annual body 10, between the wall surface of the medium diameter annular hole 8a of the sleeve 7 and the outer diameter surface of the annular body 10, and between the upper surface 11a (inner end surface) of the counter plate 11 that is fitted into the sleeve 7 and the end surface 10b of the annular body 10 and the end surface 12b of the shaft body 12a, so that the rotating portion can rotate without touching the stationary portion, thereby forming a fluid dynamic bearing.
In FIG. 22, G07 designates an axial distance of the gap formed between the end surface 10b of the annular body 10 and the upper surface 11a of the counter plate 11 when the rotor 4 (shaft 12) is rotated at a specified rotational speed.
FIG. 21 shows the state of the end portion of the shaft when the spindle motor 1 is oriented in such a manner that the counter plate 11 faces downward when the rotation of the shaft 12 is stopped and remained at rest.
In FIG. 21, loads of the hub 32, the yolk 41, and the magnet 5 assembled to the shaft 12 shown in FIG. 20 are applied downward, and thus the shaft 12 on which the annular body 10 is fitted moves downward, whereby the end surface 10b of the annular body 10 touches the upper surface 11a of the counter plate 11 via a thin fluid layer. Since the fluid layer interposed between the upper surface 11a of the counter plate 11 and the end surface 10b of the annular body 10 is extremely thin, a gap G17 between the upper surface 11a of the counter plate 11 and the end surface 10b of the annular body 10 becomes extremely small value, or otherwise they may touch each other.
In the spindle motor 1, as shown in FIG. 20, when the shaft 12 is oriented in the vertical direction and disposed on the counter plate 11, a load is applied to the lower end of the shaft 12, and thus when an impact or vibrations is applied, the fluid layer on the contact surface is susceptible to mechanical damages such as breakage or scratch.
For example, when rotation of the shaft 12 is started, so-called fluid circularity blocking action is effected because circulation of a fluid is slow due to narrow gap G17. As a consequent, the fluid layer cannot be formed quickly, and thus the body of revolution (shaft 12) cannot rise quickly or sufficiently, which may result in difficulty in performing the function of the fluid layer as a fluid dynamic bearing. In a state where the shaft 12 is not rotating, there is no rising action effected by the fluid dynamic pressure, and thus the lower end surface of the shaft 12 (the end surface 10b of the annular body 10) touches the upper surface 11a of the counter plate 11 as shown in FIG. 21, which results in scratch on both contact surfaces.
Especially, during transportation or handling, it is susceptible to a large impact. In such a case, damages on the contact surface may increase and may cause failure in the performance of the apparatus.